Tech Tip:When is a passive DI a better choice?


by Peter Janis, President of C-Tec

 

 

People often ask, "what direct box should I use, active or passive?" Unfortunately, the answer is not black or white, but shades of grey. Let me try to make it easy.

 

Radial passive direct boxes like the JDI and ProDI have the following benefits:

 

  • Excellent noise rejection from RF and magnetic fields

  • Can handle huge transients without distortion

  • Very low phase distortion - particularly in the low end

  • Tend to smooth out the transients, producing a warmer tone

 

Passive direct boxes are generally recommended when interfacing AC-powered equipment such as keyboards, drum machines, and DJ mixers, as these can often cause ground loops. These AC-powered devices are usually equipped with buffers (built-in preamps) and therefore are not subject to loading which can occur on older vintage bass guitars. Most new acoustic guitars and basses have active preamps (buffers) making them perfectly suitable for use with passive DIs.

 

Radial active direct boxes like the J48 and JDV have the following benefits:

 

  1. High-input impedance to reduce loading - important on old Fender basses and piezo pickups

 

  1. Excellent transient response to handle dynamics without choking

  2. The J48 has an isolation transformer in the switching power supply to reduce ground loops

  3. The JDV employs a feed-forward (no feedback loops) class A circuit for most natural tone - truly in a class of its own

  4. Active DIs tend to have a broader response and more air in the sound

 

Active direct boxes tend to be the first choice with acoustic guitars as active DIs tend to have more "reach" over passive units. This is analagous to comparing a dynamic mic to a condenser mic. Condenser mics have more sparkle which can be good... or sometimes undesirable. The important point here is that an active direct box is in fact a preamp that can enhance the sound. And just like mic preamps, you can get cheap ones for $59 and you can get great ones for $5,000. The difference between the two is generally headroom and overall sonic performance.

 

 

If you compare mixing consoles like a $500 Mackie to a $250,000 Neve, the Mackie has a working rail voltage of about 14 volts while the Neve will be equipped with around 24 volts. More headroom means that you get less distortion. In DI boxes that are AC powered like the Radial JDV, more headroom can be obtained by increasing the size of the power supply. The JDV has a 500mA - 42 volt power supply that can yield 21 watts of available power and produces 30 volt rails.

 

 

When using 48V phantom power as the sole power source, the available horsepower is limited to 5mA and 48V phantom. This only yields 1/4 watt of power - not much headroom for that DI. (Keep in mind phantom power was never intended for direct boxes, it was designed in the 1960s by Mr. Georg Neumann to provide a 48V charge to the plates on his famous capacitive FET microphones.) To get around this limitation, the Radial J48 follows the same design philosophy as used in today's new generation of lightweight power amplifiers whereby digital switching is employed to step up the rail voltage. A typical direct box will only yield 2 or 3 volt rails while the J48 boasts almost 9 volts or 3 times more. This allows the J48 to be used with high output instruments without choking.

 

Bottom line: it comes down to preference.... If you want more reach - go active (J48 or JDV) If you want less noise - go passive (JDI or ProDI)